A THERMOGRAVIMETRIC STUDY OF THE DECOMPOSITION OF UO3 TO U3O8 IN RELATION TO THE CONDITIONS OF PRETREATMENT OF AMMONIUM URANATE

Ammonium uranate (AU) was precipitated from uranyl nitrate with ammoni a gas and dried without washing, or washed with water, methanol, and/o r methanol-acetone, and dried. On the basis of the weight lost at 250- degrees-C (dehydration) and at 250-degrees-410-degrees-C (loss of NH3) , the chemical formula was UO3.xNH3.yH2O (x = 0.77-1.121, y = 0.82-1.1 4). Four DTG stages of decomposition appeared, and first-order kinetic analysis of the first three TG waves gave E(a) = 69, 86 and 161 kJ.mo l-1, respectively. Dissociation Of UO3 to U3O8 at 600-degrees-700-degr ees-C was associated with an E(diss) of 355-492 kJ.mol-1, dependent on the mode of washing the AU. Precalcination of the AU (18 h) led to an increase in E(diss) from 421 to 649 kJ.mol-1 on a temperature increas e from 100-degrees to 300-degrees-C, followed by a decrease on further temperature rise to 327 kJ.mol-1. For the production of U3O8, washing of AU with water and drying at 50-degrees-C, and/or precalcination at and beyond 350-degrees-C, is recommended.